Slurpee marks its unofficial birthday on July 11 with freebies. / 7-Eleven

by Bruce Horovitz, USA TODAY

by Bruce Horovitz, USA TODAY

The free Slurpee isn't just growing in hype this year - it's growing in size.

Almost twice the size.

7-Eleven, the nation's largest convenience store chain, is junking the 7.11-ounce Slurpee that it's given away for the past decade by the millions on Slurpee's unofficial birthday - July 11 - and will replace it on Thursday, during this year's Free Slurpee Day, with 12-ounce Slurpees.

For those counting, that's 42% more free Slurpee, says Laura Gordon, vice president of marketing and brand innovation. She estimates that the chain will give away about 7 million Slurpees on Thursday at a retail value of $7.6 million. "It helps us reconnect with people who love the Slurpee," she says.

The real reason for the larger-size freebie: less mess. "Moms asked us for cups with lids," says Gordon, and unlike the 7.11-ounce cups, the conventional, 12-ounce Slurpees come with lids. "That means less mess in the car."

Free Slurpee Day is the sugary, ice-laden beverage's biggest marketing push of the year - coming near the peak of the hot summer season when demand is at a zenith. The chain has been hyping the promotion on Facebook since March 11. It's pushed out gobs of Instagram photos. It's even selling Slurpee-branded merchandise at some locations during the promotion, including neon-colored Slurpee sunglasses and flip-flops that say Slurpee on the bottom.

Arguably, the 46-year-old Slurpee has more consumer cachet than does the 7-Eleven chain itself. If you're keeping score digitally, Slurpee has more than 5 million Facebook fans, vs. 7-Eleven's 3 million.

"Slurpee is the back-to-the-future part of our culture," says brand guru Allen Adamson. "For teens on a budget, 'free' is a great motivator - especially on a hot, summer day when they have nothing else to do."

While Gordon says that the promotion is all about saying thank-you to the chain's customers, she concedes there's far more at stake. "It's Slurpee's biggest marketing tool," she says. "It reminds people about Slurpee - but thanks them at the same time."

Not to forget: It's a sales bonanza, too.

Even though small Slurpees are free during the promotion - which runs from 11 a.m. to 7 p.m. local time - overall Slurpee sales are expected to jump as much as 40% on Thursday, compared with a typical July day, says Gordon.